The present invention relates to a sound amplifier system especially adapted for use with musical instruments, and in particular electric guitars.
The invention is directed to the problems encountered with sound amplifier systems often referred to as single-unit vacuum tube amplifiers. This type of system includes a housing or cabinet which supports a loudspeaker means and an amplifier means as an integral unit wherein the components are mechanically interconnected with one another. The amplifier means in this type of system includes conventional vacuum tubes which are incorporated in the low level preamplifier and power amplifier portions of the amplifier means.
Sound amplifier systems utilized with electric guitars are usually operated at nearly maximum gain in order to provide the desired tube-type distortion that is so popular in contemporary music. The requirement for high gain coupled with the high efficiency loudspeakers in current use and the generally poor quality of vacuum tubes which are now mostly imported creates a tremendous problem referred to as "tube microphonics". This is actually spurious mechanical noise generated within the vacuum tubes during operation of the sound system.
Conventional vacuum tubes employed in this type of sound system include internal elements including the usual cathode, grid and plate. These elements are concentrically disposed with respect to the longitudinal axis of the tube, the construction being such that the tube has its maximum strength and rigidity along such longitudinal axis. Any movement of a tube element or its respective support structures with respect to any other element is amplified and passes through the loudspeaker as an objectionable noise and/or high pitched squeal, referred to as tube feedback.
Movement of the tube elements is caused by motion of the loudspeaker during operation of the system. The loudspeaker is mechanically coupled to the vacuum tubes through the housing and chassis of the amplifier means. In addition, the loudspeaker produces intense air pressure waves within the housing, and these air pressure waves may impinge on the tube so as to cause movement of the tube elements. The tubes literally act like a microphone and pick up vibrations caused by motion of the loudspeaker which are transmitted both mechanically and pneumatically to the tubes.
Many efforts have been made to reduce tube microphonics in this type of sound system, but have been unsatisfactory. In present day sound systems, the loudspeakers are mounted within an associated housing so that the main axis of motion of each of the loudspeakers is disposed in a substantially horizontal position. The vacuum tubes of the amplifier means are mounted such that longitudinal axes thereof extend substantially vertically, or substantially perpendicular to the main axis of motion of the loudspeakers of the system.
The prior art has taught that the vacuum tubes should be mounted with the longitudinal axes thereof disposed substantially vertically to obtain convection cooling thereof due to the so-called "chimney effect" wherein the hot air rising off of the tubes is replaced by cooler air from below. Additionally, by having the tubes oriented in this manner, it is much simpler to remove and replace the vacuum tubes when necessary.
It has been discovered that the orientation of the vacuum tubes in the prior as discussed above resulted in vibrations being coupled to the vacuum tubes from the moving loudspeaker both mechanically and pneumatically in a direction substantially perpendicular to the longitudinal axes of the vacuum tubes. Since the tubes do not have great strength and rigidity in a direction perpendicular to the longitudinal axes thereof, excessive vibrations were induced in the internal elements of the tubes, thereby causing undesired tube microphonics.
Although not previously recognized, this is true when the vacuum tubes are disposed with the longitudinal axes thereof disposed substantially vertically and wherein the tubes extend into the path of the air pressure waves generated by the loudspeaker. The tubes in modern sound systems of this type are disposed behind the speaker and in the path of the air pressure waves which impinge thereon in a direction generally perpendicular to the longitudinal axes of the tubes. Furthermore, the vibrations of the speaker which are transmitted mechanically through the housing are constrained to be disposed in a substantially horizontal direction since the housing is supported on a horizontal surface. These mechanically transmitted vibrations accordingly occur in a direction disposed substantially perpendicular to the axis of the tubes.
In a prior art arrangement as shown in United States Pat. No. 2,231,235, the vacuum tubes of the amplifier means are disposed horizontally rather than vertically when the sound system is in operative position. However, vibrations imparted to the vacuum tubes due to motion of the loudspeaker are in a direction substantially perpendicular to the longitudinal axes of the tubes, thereby causing excessive vibration of the tube elements and undesired tube microphonics.